This invention relates to a three stage process for producing producer gas from biomass. Producer gas is a combustible mixture of nitrogen, carbon monoxide, carbon dioxide, hydrogen and methane. There are several known methods of obtaining producer gas.
The easiest way to gasify biomass is to charge fuelwood into a generator from the top and insert the air for gasification from the bottom (updraft). Air and fuel react in a countercurrent process. The thus obtained producer gas contains a lot of tar, phenols, acids, so that the purification is complicated and waste water management is very costly. To solve the waste water treatment, gasifiers were proposed where fuel and air is charged from the top (down draft gasification) and fuel and air flow in a concurrent manner. The gas is drafted from the bottom. But this type of gasifier needs dried hard wood with definite edge lengths to achieve satisfactory results. Therefore, most of the available wood wastes products cannot be gasified. Furthermore, scaling up is difficult and the purity of the gas is not ensured.
Approximately, in the last century a combination of an updraft gasifier and a second coke gasifier was proposed, in order to crack the tars, phenols, acids, and to improve the conversion of carbon dioxide to carbon monoxide and equalize the gas quality. But the demand for coke was rather high and the process control unsatisfactory.
In U.S. Pat. No. 927 418, B. Loomis and H. Pettibone disclose a process of manufacturing gas from wood by integrating a wood gasifier and a coke gasifier for a downdraft flow. Two gasifiers are connected at the top by a pipe in parallel. A layer of coke is placed above a grate and a layer of wood is placed over the coke. Wood is charged to the reactor on top of the coke beds and burned. Air is drawn up or down into the wood to cause its combustion. The wood ashes eventually plug the interstices in the body of coke and the coke must be cleaned by steam under pressure to keep it in good condition. Periodically the coke must be completely removed from the reactor in order to clean out the wood ashes. According to Loomis and Pettibone a good gas can be obtained but no detailed results on the tar and phenol content in the gas are disclosed. The process conditions are not stable because they change in proportion to the amount of wood being charged and the amount of air being drawn into the reactor. The temperature of the cracking or burning process cannot be controlled to the necessary temperature range and the residence time is not high enough for total gas reforming. The quality of the gas, therefore, varies and the tar and phenol content of the gas is likely to be quite high. Furthermore, the known process of manufacturing gas is a batch process have the following disadvantages: the process is unstable and unpredictable there is rapid consumption of coke, difficulty in replenishing the supply of coke to the reactor, and the requirement of cleaning the reactor.
During the past ten years biomass gasification in fluidized beds were tested but the results were not satisfying due to small fuel content in the fluidized bed and a low efficiency of gasification. The fuel must be very dry and only some kinds of biomass can be used. On the basis of today's calculation feasibility can only be expected for gasifiers with a performance higher than 10 MW.
As prior art, reference is also made to the so-called Reichelt purge gas process (German Patents 666,387, 712,290 and 744,135), as well as to the SIFIC process (German Patent 763,915). According to the two aforementioned processes, carbonization takes place in retorts, which are continuously or discontinuously charged with biomass. The extraction products of interest here are the condensable constituents of the organic vapors. The heat required for producing the carbonization gas is obtained by burning at least part of the residual gas.
Finally, reference is also made to the Kiener process, in which a coke gasifier is connected downstream of a low temperature carbonization gasifier and a particularly advantageous power utilization is achieved in that the producer gas is used for operating the gas motors, while the low temperature carbonization gasification takes place by means of the motor exhaust gases in a carbonization drum. As a result of the closed system, the process operates in a manner which is not prejudicial or harmful to the environment, because, except for the condensate water obtained, all the toxic compounds formed, such as, e.g., heavy metal compounds, are left behind in the low temperature carbonization coke. However, it is a disadvantage of this process that the carbonization drum is heated from the outside. Another disadvantage of this process is the interlinked insensitive and time-consuming dependence between the charging of the reactor on the one hand and the gas requirement on the other.
Therefore, it is on object of this invention to provide a process for producing producer gas from almost all biomasses which has stable and predictable process reaction conditions with a high efficiency resulting in a gas of good quality which is free from tars and phenols. Other objects will become clear throughout the course of the description provided below.